After serious thinking The Natural Philosopher wrote :
Looks like a white filled *polyester* resin.

Essentially nicer looking car body filler.

As I said, 'Milliput' is the *epoxy* equivalent.

https://www.ebay.co.uk/itm/MILLIPUT-...r/222871502626

I have used the standard milliput before and car body filler, this was
much, much finer than both of those. It ran almost like a gloss oil
paint and produced a shiny surface if just allowed to set - though it
seems to advise laying some (provided) plastic on top, whilst it set, I
didn't use it.

I deliberately smudged some onto a chrome pipe, just to see how well it
stuck. It is certainly much tougher than gloss paint to remove, it will
need to be 'chiselled off'.

I needed it for an enamelled pressed steel bath, where I had drilled a
hole to allow a pipe to pass through (bad idea). It had gradually
rusted out from the hole over the years and I had repaired it once
already with phosphoric acid and white car touch up paint, from my old
Granada.